Method of preparing metal foil/graphite fiber/epoxy resin laminates

ABSTRACT

Novel metallic foil/graphite fiber/epoxy laminates with compound curvature are fabricated through a unique fabrication process which includes the steps of preparing the laminate in the prepreg stage with the metallic foil layers overlapping the graphite layers, clamping the overlapped portion of the foil layers and pulling the laminate down over a mandrel of desired compound curvature, and curing the resulting curved laminate.

United States Patent 1 Zorowski et al.

[ 1 Jan. 23, 1973 METHOD OF PREPARING METAL FOIL/GRAPHITE FIBER/EPOXYRESIN LAMINATES Inventors: Carl F. Zorowski; John D. Calfee,

both of Raleigh, NC.

Assignee: Monsanto Company, St. Louis, Mo.

Filed: Sept. 17, 1970 Appl. No.: 72,991

U.S. Cl. ..29/445, 29/446, 29/475, 156/163, 156/165, 156/242 Int. Cl...B23p 9/00 Field of Search ..29/446, 460, 475, 445; 156/242, 163

References Cited UNITED STATES PATENTS Woodard ..29/445 UX Cameron eta1. ..29/445 X 3,004,324 10/1961 Macomber ..29/460 X 3,052,021 9/1962Needham ..29/445 X 3,200,026 8/1965 Brown ..156/ 163 X 3,480,501 11/1969Burch ..l56/245 X 3,655,472 4/1972 Chandler ..l56/163 X PrimaryExaminer-Charlie T. Moon Attorney-Vance A. Smith, Russell E. Weinkauf,John D. Upham and Neal E. Willis [57] ABSTRACT Novel metallicfoil/graphite fiber/epoxy laminates with compound curvature arefabricated through a unique fabrication process which includes the stepsof preparing the laminate in the. prepreg stage with the metallic foillayers overlapping the graphite layers, clamping the overlapped portionof the foil layers and pulling the laminate down over a mandrel ofdesired compound curvature, and curing the resulting curved laminate.

3 Claims, 3 Drawing Figures PATENTEDJAH 23 I975 FIG.

FIG. 3.

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METHOD OF PREPARING METAL FOIL/GRAPHITE FIBER/EPOXY RESIN LAMINATESCROSS-REFERENCES TO RELATED INVENTIONS This application is related tocommonly assigned and copending applications Ser. No. 889,395 filed Dec.31, 1969, now abandoned, of John D. Calfee and David A. Zaukelies; Ser.No. 17,099 filed Mar. 6, 1970, now abandoned, of David A. Zaukelies; andSer. No. 48,961 filed June 23, I970 ofJohn D. Calfee.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a metallic foil/graphite fiber/epoxy laminatestructure having compound curvature and a method of producing the same.

2. Discussion of the Prior Art In the referenced copending applications,Ser. No. 889,395 and Ser. No. 17,099, detailed descriptions are given ofmetallic foil/graphite fiber/epoxy laminates which display markedincreases in transverse strengths and moduli over all graphitefiber/epoxy laminates, the latter being weak in directions transverse tothe fiber direction. Referenced copending application Ser. No. 48,961describes a similar laminate with improved impact resistance due to theaddition and critical positioning of layers of highly resilient fiberssuch as glass fibers. The laminates as described and claimed in thereferenced applications are generally suitable for use in aircraft andthe like where high strength/high modulus, low density materials havingthicknesses between about 20 to 60 mils are needed.

The use of the laminates, however, has been limited to flat structuresor structures with simple curvatures. Many portions of an aircraft, thenose section, for example, have curvatures both around and along thelongitudinal dimension, such curvature is hereinafter called compoundcurvature. Attempts to fabricate shapes having compound curvature fromlaminates described in the copending applications heretofore have notmet with success. The foil itself is easily bent, drawn, or stretchedwhen constrained to conform to a shape with desired compound curvature.On the other hand, graphite fiber layers are relatively inextensible andtend to break, separate, and abraid when such constraints are imposed.

It is therefore an object of the present invention to provide for aprocess by which metallic foil/graphite fiber/epoxy laminates havingcompound curvature may be made.

It is another object of the present invention to provide for a metallicfoil/graphite fiber/epoxy laminate having compound curvature.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features believedcharacteristic of the present invention are set forth in the appendedclaims. The invention with further objects and advantages thereof may bebest understood by reference to the following description taken inconnection with the accompanying drawings in which:

FIG. 1 is a schematic of a typical metallic foil/graphite fiber/epoxyresin prepreg which may be used in accordance with the presentinvention.

FIG. 2 is a perspective view of an apparatus which may be utilized topractice a preferred feature of the present invention.

FIG. 3 is a perspective view of a prepreg similar to that illustrated inFIG. 1 being drawn over a mandrel of selected compound curvature.

DESCRIPTION FIG. 1 illustrates schematically a multilayer'metallicfoil/graphite fiber/epoxy resin prepreg 10. As depicted, metallic foillayers 11 overlap graphite fiber/epoxy resin layers 12. Glass (or otherresilient) fiber layers may be added as desired. As explained below, theoverlap portions provide a means by which the foil layers may be drawnwithout significantly affecting the relatively non-stretchable graphitefibers. The metallic foil may be any metallic material such as, forexample, aluminum, titanium, steel, or any alloy thereof.

FIG. 2 illustrates an apparatus which may be used to form a metallicfoil/graphite fiber/epoxy laminate with compound curvature from aprepreg such as that depicted in FIG. 1. Base plate 20 is attached tofour upright members 21, each of which fixedly support a thrust bolt 22.Each pair of thrust bolts 22 movably support a clamping rail 23. Nuts 24positioned on thrust bolts 22 provide the movement of rails 23.Resilient means, such as C clamps 25, maintain rails 23 movably closed.

In operation, a mandrel or mold 26 having the compound curvature desiredis centered on base plate 20 between upright members 21. A metallicfoil/graphite fiber/epoxy resin prepreg 27 (of a structure similar tothat illustrated by FIG. 1) is loosely clamped on the extended portionof metallic foil by clamping rails 23. A lubricant, such as polyethylenedispersed in a grease, is placed on mold 26 to reduce friction betweenthe foil and mandrel thereby preventing galling, scratching or marringof the foil. Base plate 20 may be adjustable in height so as to ensurethat the plane extending between clamping lines of clamping rails 23 isat the desired level above plate 20.

As shown in the perspective schematic of FIG. 3, increasing tension issupplied to the metallic foil by moving clamping rails 23 in thedirection indicated by the arrows. The increasing tension causes themetallic foil to draw out and down over mandrel 26 while concurrentlycausing the graphite fibers to distort in conformity with the compoundcurvature of mandrel 26. It is necessary that the resin be present inamount sufficient to wet the surface of the foil and fibers during thedrawing process. The laminate may then be cured while maintaining thetension on the metallic foil. It is also desirable that the metallicfoil be partially annealed so as to provide sufficient ductility fordrawing.

Reference is now made to the following example which is illustrative ofa preferred technique of preparing prepregs to be utilized in accordancewith the present invention and describes in greater detail thestretching and deformation of the prepreg.

EXAMPLE Although there are various techniques by which multi-layermetallic foil/graphite resin laminates can be made, such as, forexample, laying graphite fiber prepreg tapes directly on metallic foilheld under tension, the preferred process is that which is described inthe referenced copending application, Ser. No. 17,099.

Briefly, a sample of commercially available graphite yarn, for example,HMG-SO obtainable from the Hitco Company, may be utilized to prepare theprepreg sheet of this example. The yarn derived from a rayon precursor,was a l440-filament 2-ply, 4 turns per inch yarn with a modulus of about50 X 10 lbs/sq. inch and a tensile strength of 300 X- 10 lbs/sq. inch.The yarn was unreeled from a bobbin (power driven to minimize tension)and passed at about 30 feet per minute through a heating oven raised toa temperature of about l250-l 350C, and filled with nitrogen containingabout 0.3 percent oxygen. After cooling in another chamber withnitrogen, the yarn was wrapped under about 80 grams tension onto alathe-driven 13.5 inch diameter drum wrapped with a solventcleaned bailof commercially available 5 mil aluminum foil.

A resin having good adhesive characteristics, such as commerciallyavailable bisphenol, an adhesive-type of epoxy resin formulation(American Cynamide BP-907, containing a catalytic curing agent) wasadded continuously from a nozzle set at the point of juncture of theyarn and drum. The resin loading, after evaporation of the solvent, wasabout 40 percent by volume. A transverse mechanism synchronized with thelathe, slowly laid the impregnated yarn in a single spiral layer,sideby-side across the face of the drum. The layer of resin and fiberwas air-dryed on the drum to a solvent content of about 1 percent andthen removed from the drum. To ensure proper wetting of foil and fibersurfaces during drawing of the foil, it is necessary that the resin beabout 40 volume percent or greater. The overlap of the aluminum foilprovided the extended portion of the foil on one side of the prepreg.The extended portion on the opposite side was provided by pulling thegraphite fiber/epoxy resin layer away from the aluminum foil so as toexpose the foil a distance sufficient for clamping. The pulled awayportion of graphite fiber/epoxy resin layer was then cut away orotherwise removed.

Additional layers of 8 mil graphite fiber/epoxy resin prepreg were addedto this structure, the resulting graphite fiber orientation being +45,-45, and 0, respectively. Another mil aluminum foil'layer was added tothe reverse side of the structure with portions extending beyond thegraphite fiber/epoxy resin layers underneath.

The laminate prepreg was then placed in an apparatus similar to thatillustrated by FIG. 2. A mandrel of predetermined compound curvature wasplaced upon the base plate beneath the laminate prepreg. The metallicfoil was placed under tension sufficient to cause the metallic foil tobe stretch drawn down over the mandrel while concurrently causing thegraphite fibers to conform to the mandrels compound shape. The laminateprepreg was then cured at about 350F for approximately 1 as hours whilebeing held under tension over the mandrel. The laminate was then removedfrom the stretching apparatus and inspected. An examination of thelaminate detected no delamination. Fiber orientation was also unaffectedother than to the extent that the fibers follow the compound curvatureof the mandrel.

In summary, the ObjCCtlVCS as set forth have been attained. A method offabricating metallic foil/graphite fiber/epoxy laminates with compoundcurvature heretofore unattainable has been demonstrated. The novellaminates so fabricated are suitable for applications where curvedstructures of high strengths/moduli and low density are needed. Thus, itwill be apparent to those skilled in the art in light of this disclosurethat variations and changes may be made which do not depart from thefull intended scope of the invention as defined by the following claims.

What is claimed is:

a. Preparing a metal foil/graphite fiber/resin epoxy laminate with themetal foil layers extending beyond the graphite fiber/resin epoxy layersin the longitudinal direction;

. clamping under tension the extension of the metal foil layers;

. drawing the metal foil layers around a mold having a predeterminedcompound curvature, the resin content of the prepreg being sufficient topermit resin flow for essentially complete wetting of the graphitefibers and surfaces of the metal foil layers and for filling theinterstices in the graphite fiber layers during drawing; and

curing the stretched metal foil/graphite fiber/resin epoxy prepreg.

2. The process of claim 1 wherein the epoxy resin content is greaterthan about 40 volume percent.

3. The process of claim 2 wherein the epoxy resin content is about 60volume percent.

2. The process of claim 1 wherein the epoxy resin content is greaterthan about 40 volume percent.
 3. The process of claim 2 wherein theepoxy resin content is about 60 volume percent.